Controller method receives data posted by Ajax as null

I have a simple controller method that expects to receive a string argument:

    public class ZTest : BaseController
    {
        [HttpPost]
        public async Task<ActionResult> Test(string msg)
            {
            // BREAKPOINT   msg is always null here
            return Json(ModelState.ToDataSourceResult());
            }
    }

A basic Ajax request posts data to the controller method:

            $.ajax({
                type: 'POST',
                url: '@Url.Action("Test", "ZTest")',
                dataType: "json",
                contentType: "application/json; charset=utf-8",
                data: JSON.stringify({ msg: "Hello World" }),
                    success: function (data) {
                    if (data.exception) {
                        Swal.fire("Error", data.message, "error");
                    }
                    else {
                        Swal.fire("Success", "message", "success");
                    }
                }
            });

Why would this simple example fail?

I created this simple MRE while trying to troubleshoot a more involved example.

I have experimented with adding and removing the [HttpPost] decoration, prepending the string msg variable with [FromBody], and removing the Ajax properties dataType and contentType.

Solution

Now we're using contentType: "application/json; charset=utf-8",this means that the JSON object sent will be bound to the object（Message） in C#.

Here is the test code in my side

    [HttpPost]
    public IActionResult Test([FromBody] Message msg)
    {
        return Json(msg);
    }
    public class Message
    {
        public string? msg { get; set; }
    }

Test Result